As known, one of the requirements for microwave oscillators is a frequency stability irregardless of temperature or other circuit parameter variations, for example, a supply voltage variation. The most important perturbations are caused by temperature variations of the active device, generally consisting of a junction transfer or of an FET.
Such variations cause a considerable variation of electric parameters (.beta. or .mu.), which cannot be directly compensated for, hence a stabilizing element outside the device is necessary.
A widely-used method uses a high-stability resonator with high quality factor (Q) placed at a suitable circuit point.
As is known, the oscillation frequency is determined by zero reactance conditions in the circuit under working conditions and near the resonance frequency of the resonator; reactance variations of the active device due to temperature variations are compensated for by reactance variations of the high-Q element, originated by a low frequency-shift. In fact, this type of resonator yields considerable reactance variations in the vicinity of the resonance frequency, allowing zero reactance conditions to be gained again.
Nowadays the resonant high Q element generally consists of a dielectric resonator. An application of such an element is shown, e.g. in U.S. Pat. No. 4,149,127. There a dielectric resonator stabilized FET microstrip oscillator is described, the resonator being placed on the gate line. The feedback for starting the oscillations is obtained by a suitable-length line trunk connected to the source and terminated by a short circuit.
This circuit can supply a highly stable oscillation frequency, but may be insufficiently protected against the danger of parasitic oscillations chiefly at low frequencies, since the impedance viewed from the source is a pure reactance. To overcome this inconvenient, a resistive component ought to be introduced which intervenes only outside the operating frequency band, thus reducing the whole circuit gain and hence the degree of positive feedback.
A recent embodiment of a microwave oscillator, described in the paper entitled "Efficient low-noise three port X-band FET oscillator using two dielectric resonators", Proceedings of International Microwave Symposium--1982--Dallas (Tex.), realizes this condition by using a second resonating element, identical to the stabilizing element. This is placed along a line outgoing from FET source, which is no longer short-circuited, but is terminated by a 50 ohm adapted resistive load. The resonating element coupled to the source line operates as a parallel-type resonator placed in series and hence at resonance the line becomes an open circuit. By suitably positioning the resonator, the desired reactance can be obtained at the source.
Far from resonance the resonator behaves as a short circuit, that is why the line results terminated into its characteristic impedance and a pure resistive load is obtained at the source. As a consequence parasitic oscillations are rare.
However such a circuit presents two disadvantages. Firstly, the resonator placed along the source line realizes at the resonance an imperfect open-circuit, as a non-negligible resistive component remains even for high coupling values. This results in a lower degree of positive feedback and hence in lower output power.
Secondly, the presence of two high-Q and hence highly-limited-band resonators renders the oscillator adjustment very critical. In fact the oscillation condition takes place only when the two resonators are nearly exactly synchronous, while a small resonator displacement in the direction perpendicular to the line is sufficient to modify its resonance frequency.